Targetting of protein phosphatase 1 to the sarcoplasmic reticulum of rabbit skeletal muscle by a protein that is very similar or identical to the G subunit that directs the enzyme to glycogen

The amount of protein phosphatase 1 (PP1) activity in rabbit skeletal muscle associated with membranes (predominantly sarcoplasmic reticulum) is similar to that bound to glycogen‐protein particles. Membrane‐vesicleassociated (sarcovesicular) PP1 can be solubilised with 0.5% Triton X‐100 (but not 0.5 M NaCl) and is complexed to a protein that is structurally and functionally very similar or identical to the G subunit which targets PP1 to glycogen‐protein particles. This conclusion is based on immunoblotting and immunotitration experiments using two different preparations of G‐subunit‐specific antibodies, binding of Triton‐solubilised sarcovesicular enzyme to glycogen, stimulation of phosphorylase phosphatase activity by glycogen, phosphorylation of the same tryptic peptides by cyclic‐AMP‐dependent protein kinase (A‐kinase) and release of catalytic subunit following phosphorylation by A‐kinase. Membrane‐association is not mediated via glycogen because sarcovesicular PP1 is (1) not released by digestion with α‐amylase or at dilutions which fully dissociate the glycogen‐bound enzyme, and (2) is solubilised by Triton X‐100 (whereas glycogen‐associated PP1 is not). These findings demonstrate that sarcovesicular PP1 is highly homologous to, or the same as, glycogen‐associated PP1G and raises the possibility that a common targetting subunit may direct PP1 to different subcellular locations.